The present invention relates to pistons for hydraulic pumps and motors. More particularly, this invention relates to a filling for hydraulic pistons used in pumps and motors. The filled piston of this invention increases the efficiency of the pump or motor at a reasonable cost.
A known technique for reducing the amount of oil that is contained within a hydraulic piston is to fill the normally hollow piston with a solid material. This reduces the amount of oil contained within the piston. The oil within the piston must be compressed during each revolution or pumping cycle.
Hollow piston constructions have been found to produce adverse side effects due mainly to the compressibility of the oil which fills the piston cavity. The compressibility of the fluid has a marked effect upon the overall efficiency of the unit, and also produces cavitation, erosion, noise and undesirable moments on the swashplate mechanism when used in an axial piston type of pump or motor.
There are currently at least three known types of "filled" hollow pistons: welded pistons, solid pistons, and plastic-filled pistons. Welded pistons are costly to manufacture because of the welding process. Welded pistons also require that a drilled orifice be provided through the unit for lubrication of the slipper running face. These drilled holes are usually relatively long and small in diameter. Therefore, the drilling process is typically quite difficult and expensive.
Solid pistons also reduce the oil volume. However, solid pistons are much heavier than their hollow counterparts and therefore reduce the speed capability of the hydraulic unit. Similar to welded pistons, solid pistons have a small hole therethrough which requires an expensive drilling operation to ensure lubrication for the slipper running face.
Filling the pistons by pouring a liquid plastic material into them has also been tried. When solidified, the plastic has a bulk modulus greater than that of oil. This method has proven to be costly, and it has been difficult to reliably retain the material within the piston or adhere it to the piston wall. Many plastics do not meet the bulk modulus requirement.
The oil volume in each piston bore is compressed to the operating pressure during each rotation of the cylinder block. Oil is compressible and takes energy to compress. This results in energy losses for units that do not have the piston filled. Providing this reduced volume can improve the efficiency but usually requires a higher cost to produce.
Another problem with the hollow pistons is the variation in control moments with changes in block rotational speed. Direct displacement (non servo) units typically do not have pistons with reduced volume because of the higher cost. Since direct displacement units do not have a servo to control the swashplate, the operator feels the control moments.
Pistons with low volume can be manufactured as welded pistons or as solid pistons. Both achieve the performance improvements but the welded is costly to produce and the solid pistons are much heavier which reduces the maximum speed at which they can operate due to higher block tipping forces and higher centrifugal forces causing piston burn.
It has been difficult to adapt the conventional "filled" pistons described above to lower-pressure hydraulic units. Thus, the lower-pressure hydraulic units do not get the benefit of the reduced oil volume because they are typically lower-cost units, and the market will not tolerate the additional cost of the non-hollow pistons.
Therefore, a principal object of the invention is to provide a lightweight insert in the piston retained by cold forming the piston material around the insert.
A further objective of the invention is the provision of a lightweight piston filling which can be produced without expensive casting or extrusion methods.
A further objective of the invention is to provide a lightweight piston which is low cost and which reduces oil compressibility.
These and other objectives will be apparent to those skilled in the art.